1. Field of the Invention
The present invention relates to a hydrogen gas sensor, and more particularly, to a hydrogen gas sensor suitable for measuring the hydrogen concentration of a fuel gas used for fuel cells.
2. Description of the Related Art
In view of the issue of global-scale environmental deterioration, fuel cells, which are clean and efficient power sources, have recently become the subject of active studies. Among fuel cells, a polymer electrolyte fuel cell (PEFC) is expected to be suitable for vehicle use due to its advantages, including low operation temperature and high output density. In this case, a reformed gas obtained from methanol or the like is advantageously used as a fuel gas. Further, in order to improve efficiency and other parameters of performance, a gas sensor capable of directly measuring hydrogen concentration of the reformed gas is needed.
Since such a hydrogen gas sensor is used in a hydrogen-rich atmosphere, the operation temperature of the gas sensor must be low (about 100° C. or less). Such a low-operation-temperature-type sensor is disclosed in Japanese Patent Publication (kokoku) No. 7-31153. In the sensor, a working electrode, a counter electrode, and a reference electrode are disposed on an insulating substrate, and the three electrodes are integrally covered with a gas-permeable, proton-conductive film; more specifically, “NAFION®” (trademark, product of Dupont), which is a type of fluororesin. NAFION® is a proton-conductive material capable of operating at low temperature and is used at portions of polymer electrolyte fuel cells.
The present Inventors found that when NAFION® is used as a proton-conductive layer as in the gas sensor disclosed in Japanese Patent Publication No. 7-31153, the sensor output varies depending on the H2O concentration partial pressure of a gas under measurement (hereinafter referred to as a measurement gas atmosphere), so that accurate measurement becomes difficult. Further, the present Inventors found that the above phenomena occurs because protons pass through NAFION® together with H2O molecules, and therefore, the proton conductivity varies with the H2O concentration of the measurement gas atmosphere. That is, when the proton-conductive layer is formed of NAFION®, the sensor output depends on the H2O concentration of the measurement gas atmosphere, so that the sensor output decreases greatly, especially when the H2O concentration is low.
The present Inventors further found that although porous Pt electrodes (catalysts) are generally known to exhibit high activity at low temperature (porous Pt electrodes are used, for example, in fuel cells), when such a Pt electrode is exposed to an atmosphere having a high CO concentration, CO is adsorbed on the Pt electrode, or the Pt electrode is CO-poisoned, so that the sensor output is greatly decreased.
Since many fuel cells use pressurized fuel gas in order to improve power generation efficiency, sensors used in the fuel gas are required to have a small pressure dependency. However, in the sensor described in the above-mentioned Japanese Patent Publication No. 7-31153, gas under measurement is diffused to the working electrode via the gas-permeable, proton-conductive film, so that the sensor exhibits a great pressure dependency, depending on the structure of the proton-conductive film itself, and therefore high measurement accuracy cannot be obtained.